Along with the continuous development of display devices, particularly mobile display devices, common physical keys are continuously replaced by other means in these devices, and people tend to adopt lighter and thinner, more sensitive and more fashionable virtual keys to achieve the control and input for the display devices. Moreover, along with users' higher and higher requirements on the narrow bezels of display devices, almost all the mobile phones and tablet PCs pursue for narrow bezels, so as to improve the screen-to-body ratio and hence improve user experience. However, common mobile display devices still have physical keys, e.g., VOLUME and ON/OFF, on the upper side and/or the left side and/or the right side of a bezel. As the physical keys have larger sizes, the display devices cannot further reduce the bezel width.